1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a dot matrix type liquid crystal display device adapted for displaying arbitrary shapes on a display surface.
Although description will mainly be made, taking color super homeotropic (CSH) liquid crystal display (LCD) as an example, the invention is not limited to CSH LCD. For example, it can also be applied to a homogeneous LCD.
2. Description of the Related Art
In a CSH LCD, homeotropically or substantially homeotropically oriented nematic liquid crystal molecules are electrically controlled in their birefringence to achieve a desired display.
A CSH LCD without pre-tilt according to the prior art will be described, referring to FIGS. 2A to 2D.
As shown in FIG. 2A, a liquid crystal molecule 10 has an elongated shape, and has a higher optical constant (refractive index) along the long axis. Further, the liquid crystal molecule has an electric bipole along a direction orthogonal to the long axis. In the "off" state where an electric field above a certain value is not applied between the electrodes, the liquid crystal molecules take a homeotropic orientation where the long axis is normal to the surface of the substrate, as shown in the left part of FIG. 2A. When an electric field E is applied normal to the substrate, there occurs a force in a direction which drives the electric dipole of the liquid crystal molecules to follow the direction of the electric field and the liquid crystal molecule 10 is tilted as shown in the right part of FIG. 2A. This angle of tilt is called tilt angle, and is of the order of 10 degrees, for example.
FIG. 2B shows an electric field distribution in the electrode crossing portion in the liquid crystal display device. A pair of glass substrates 11 and 12 are disposed parallel to face each other. On the inner surfaces of the substrate, segment electrodes 1a and 1b and common electrode 2 are formed in a crossing relation. The common electrode 2 defines the row of the dot matrix and the segment electrodes 1a and 1b define the column of the dot matrix. On the outer surfaces of the glass substrate 11 and 12, crossed polarizers 13 and 14 are disposed. When a voltage is applied between the segment electrodes 1a and 1b and the common electrode 2, an electric field is established between the electrodes. At the edge of the electrode, fringe effect due to the edge is produced. Namely, the electric force line starting from the edge of the segment electrode 1a or 1b, for examplle, is bulged towards the common electrode 2 as shown by the broken line in FIG. 2B to have a lateral or horizontal component as well as the vertical component. The liquid crystal molecule changes its tilt according to the electric field distribution.
Such distribution of the liquid crystal molecules is shown in FIG. 2C in more detail.
FIG. 2C shows schematically how the liquid crystal molecules are distributed in the electrode crossing area 5. The electrode crossing area 5 is defined between the vertical facing edges 6a and 6b and between the horizontal facing edges 7a and 7b. Since the common electrode 2 extends long in the horizontal direction, the electric force line at the facing edges 6a and 6b, starting from the edges of the segment electrodes 1a and 1b are diverged outwardly as shown by the arrow. Since the segment electrodes 1a and 1b extend long in the vertical direction, the electric force line at the horizontal facing edges 7a and 7b of the electrode crossing area 5, starting from the segment electrode are distributed to be oriented from the outside to the inside. Here, the polarization axes P1 and P2 of the crossed polarizers are disposed at angles 45 degrees slanted from both the row and the column as shown in the right part of FIG. 2C. The liquid crystal molecule has a function of rotating the polarized light when tilted from the normal direction, but those liquid crystal molecules which are slanted in the direction of the polarization axes P1 and P2 have no such function. Therefore, the light is cut off by the crossed polarizers.
Therefore, as shown in FIG. 2D, there appear crossing black lines in each cell. Namely, as shown in FIG. 2D, the electrode crossing area 5 is divided into four regions D1, D2, D3 and D4. At the crossing point X of these four regions, the liquid crystal molecules are kept normal to the substrates 11 and 12 as shown in FIG. 2D. On the black lines 8, the liquid crystal molecules are tilted in the direction of the polarization axis P1 or P2. In the four regions D1, D2, D3 and D4, the liquid crystal molecules are tilted substantially leftward, rightward, upward and downward, respectively, as shown in FIG. 2D.
In the liquid crystal display device as described above, the shapes and the areas of the four regions D1, D2, D3 and D4 divided by the crossing black lines vary according to the balance of the various condition, are not constant and differ from cell to cell.
In order to solve the varying and crossing black lines in the display, a preliminary pre-tilt angle in a certain direction may be given to the liquid crystal molecules. For example, referring to FIG. 2D, when a pre-tilt angle of not more than one degree in a direction corresponding to the region D3 is given to the liquid crystal molecules in the whole electrode crossing area, almost all the liquid crystal molecules will be tilted in the same direction when an electric field is applied. Then, the most part of the electrode crossing area 5 will be occupied by the region D3. Namely, the other three regions D1, D2 and D4 become extremely small areas in the display and the quality of display is greatly improved. When such a pre-tilt is given to control the orientation direction of the liquid crystal molecules, another problem arises.
Namely, when the most part of the electrode crossing area 5 is occupied by the region D3, and when the liquid crystal display device is observed from a direction coinciding with the long axes direction of the liquid crystal molecules in the region D3, the liquid crystal molecules lose the optical rotatary power. Then, the whole surface of the display appears black. This black hole like observation angle appears, for example, at a position about 10 degrees from the normal direction to the surface of the liquid crystal display device. The fact that this black hole phenomenon appears very near the normal direction to the surfaces causes a large problem in the performance of the display device.
According to the prior art as described above, the shapes and the positions of the crossing black lines in the display area vary uncertainly to cause a problem in the display performance, or there arises a direction near the normal to the display surface in which the display quality is extremely low.